Just posted this in the Logicprohelp forum after days of beating my head against the wall.

My mbp died and had to upgrade to a 2018, complete with all the insufferable adapters you have to buy to get a firewire connection. I could not after a dozen installs and uninstalls of the YSFW get the mbp to recognize the XS8. Here is the resolution:

First, in System Preferences>Security & Privacy>Privacy>Accessibility, you must allow the Yamaha Steinberg FW Control Panel. Next, in System Preferences>Security & Privacy>General, you must allow the system software from developer “Yamaha Corporation”.

It still took three complete uninstalls, restarts and installs after changing these settings for the driver to work, but hey, I’m up and running.